This invention relates to organic electrolyte electrochemical storage cells. Particularly, this invention relates to redox shuttle additives for both liquid organic and solid polymer electrolyte electrochemical storage cells to provide overcharge protection to the cell.
Electrochemical storage batteries of all types are susceptible to damage due to overcharging or over discharging. Overcharging of an electrochemical storage cell in a battery may be defined as charging beyond a cell's capacity, or at a rate greater than the cell's ability to accept such charge. The damage to the cell which may occur from such overcharging may include oxidation of the electrolyte leading to the production of protons which may then intercalate into the cathode material resulting in reduced cell capacity.
Protection against overcharging of a single cell, or a battery comprising a small stack of series-connected cells, may be achieved through direct monitoring (potentiometric, galvanometric, thermal, etc.), control of charging rates, and state of charge. However, for a large (typically bipolar) stack of cells of the magnitude required, for example, in batteries for use in electric vehicles, these methods are impractical due to their complexity, weight requirements, and expense. Under utilization of capacity or addition of immobile electroactive chemicals to one or more of the electrodes may provide some protection (at a considerable cost), but such techniques are ineffective against significant deterioration of capacity of a single cell within a stack, which is generally cumulative, and which may lead to a short or an open circuit.
For cells utilizing aqueous liquid electrolytes, production of oxygen and hydrogen gas at the respective electrodes provides a reversible reaction which serves to protect against overcharging, provided that the cell is a vented cell, or at least that the evolution of hydrogen and oxygen during the overcharge does not exceed the rate at which the hydrogen and oxygen recombine to form water.
For organic liquid electrolytes, a "redox shuttle" has been proposed as an approach to solving the problem of overcharging. This approach employs an electrolyte additive which is inactive under normal conditions, but which oxidizes at the positive electrode (cathode) when the cell potential exceeds the desired voltage, i.e., when the cell is in an overcharge state. The oxidized form of the shuttle additive diffuses through the cell to the negative electrode (anode) where it is reduced to its original (unoxidized) state and then the reduced form of the redox shuttle species diffuses through the cell back to the positive electrode to continue the redox cycle. The net effect is an internal shunt which prevents damage to the cell by imposing a limit on cell potential. As an example of the use of such a redox shuttle in an electrochemical cell with a liquid electrolyte, the use of metallocenes, such as ferrocene, as additives to a liquid electrolyte for overcharge protection has been suggested by Golovin et al., in "Applications of Metallocenes in Rechargeable Lithium Batteries for Overcharge Protection", Journal of the Electrochemical Society, Vol. 139, No. 1 (1992), at pp. 5-10.
The difficulty with the use of such a shuttle species, however, is that the most desired reactions are those which are difficult to ensure with a lithium organic system, i.e., a lithium cell with a solid organic polymer electrolyte. Since lithium metal is extremely reactive with organic materials, it is desirable that the shuttle only react with the lithium when the shuttle is in an oxidized state. Reactivity of lithium with organic materials is discussed by Adalbert Maercker in "Ether Cleavage with Organo-Alkali-Metal Compounds", Angew. Chem. Int. Ed. Engl. 26 (1987) pp 972-989.
Furthermore, the redox shuttle species must also be mobile enough to carry enough current through the solid polymer electrolyte between the electrodes to protect the cell. This mobility or diffusion capability must be in both directions, i.e., both when the oxidized shuttle is migrating from the cathode to the anode, and when the reduced form of the shuttle is migrating from the anode to the cathode.
Since lithium reacts with all organic species, a kinetic barrier to this reaction must be introduced. Most of all, the shuttle cannot contain functional groups that are more reactive than the electrolyte itself.
The use of redox shuttles in high voltage cathode lithium cells (4.3 V vs. Li) is even more difficult as the product of the oxidation at the cathode is a very energetic species. The intermediate may dimerize, deprotonate, or cyclize. A worse possibility is that the oxidized shuttle may remove an electron from an electrolyte species (solvent or salt) thereby acting as a catalyst for degradation of the electrolyte, creating a situation possibly worse than no shuttle at all.
It would, therefore, be desirable to provide a redox shuttle which will avoid these problems and yet be useable as a high voltage (i.e., over 4 V) shuttle. It would be further desirable if the shuttle material would be capable of modification to permit operation at various voltages. In particular, it would be desirable if one could achieve the desired overcharge protection, even at high voltages, while guiding the reactivity of the resulting shuttle species in a way that can be reversed at the other electrode, and which results in the production of a less energetic species that diffuse (in either direction) back to the other electrode, thereby reducing the risk of unwanted side reactions.